The flow of electrons from high energy reductants to lower energy oxidants in an ordered series of enzyme catalyzed reactions provides the free energy necessary to drive life processes. In the recent past the complexity of the multicenter redox enzymes involved in these reactions has been recognized and the importance of this class of proteins in the catalysis of multielectron transfer reactions throughout nature has been stressed. We propose a thorough physical characterization of the structural and mechanistic aspects of intermolecular and intramolecular electron transfer for two multicenter redox enzymes; beef heart cytochrome oxidase and Chromatium flavocytochrome c552. The former protein, which catalyzes the four electron reduction of oxygen to water using ferrous cytochrome c, 4 cyt c2 ion plus O2 plus 4H ion yields 4 cyt c3 ion plus 2H2O is of major importance in maintaining mitochondrial electron flow and ATP synthesis; the latter appears to be vital to bacterial H2S metabolism. The potential for interaction between centers and the probable existence of control mechanisms for electron flow in each of these proteins is recognized. The redox centers in these proteins (heme iron and copper in cytochrome oxidase, heme iron and flavin in C552) have physical properties (notably color and, for certain valence states, paramagnetism) which will be exploited by using a number of spectroscopic and kinetic techniques. Each of these techniques is well suited to probe one or a few of the relevant properties of these proteins. They include electron paramagnetic resonance, resonance Raman and optical absorption spectroscopies and scanning stopped flow kinetics. Moreover both proteins are particularly amenable to biochemical manipulation so a judicious combination of biochemical and physical methods is integral to our approach. Because of the complexity of these proteins we expect that mechanistic insight will come through a synthesis of the conclusions reached by the individual techniques.